Halibut fishing in Seward, AK

A writing project presentation

Leslie Gains-Germain








"We travel farther for smaller fish"

Questions of Interest

Data from Alaska Department of Fish and Game

Question 1: Is there evidence that the distance Seward charter boats travel to fish for bottomfish has changed over the years 1993-2014?

Logbook Data

Interview Data



Question 2: Is there evidence of a change in the mean length of halibut caught by Seward fishermen over the years 1994-2013?

Biological Data

Data Reference

  1. ADF&G Saltwater Logbook Database [Intranet]. 2004–2013. Anchorage, AK: Alaska Department of Fish and Game, Division of Sport Fish (cited 4/2/2015). Custom query details available upon request from ADF&G, Division of Sport Fish, Research and Technical Services.

  2. ADF&G non-confidential data. 1993-2014. Homer, AK: Alaska Department of Fish and Game, Division of Sport Fish. Personal communication, Scott Meyer.

  3. ADF&G non-confidential data. 1994-2013. Homer, AK: Alaska Department of Fish and Game, Division of Sport Fish. Personal communication, Scott Meyer.

Logbook Data 2004-2013

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Logbook: 2004-2013

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Interview Data 1993-2014

jack

  • Two days a week randomly selected for interviews
  • Effort spread across the four harbor cleaning stations
  • Each boat is asked:
    • Did you catch bottom fish today?
    • If yes, record trip type
    • Identify the primary statistical area where the majority of bottomfish were caught.


abbreviation description
H halibut exclusively
B any combination of bottomfish (halibut, rockfish, lingcod, Pacific cod, sharks)
B+S any combination of bottomfish and salmon (e.g. halibut+coho)

Interview 1993-2014

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Comparison- Cape Cleare

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Fitted Probabilities and 95% Pointwise CIs

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The odds of fishing in Cape Cleare in \(2014\) are estimated to be \(2.43\) times the odds of fishing in Cape Cleare in \(1993\), with a \(95\%\) confidence interval from \(1.22\) to \(4.92\) times.

The odds of NOT fishing in Resurrection Bay in \(2014\) are estimated to be \(2.78\) times the odds of NOT fishing in Resurrection Bay in \(1993\), with a \(95\%\) confidence interval from \(1.32\) to \(5.82\) times.

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Question 2: Is there evidence of a change in the mean length of halibut caught by Seward fishermen over the years 1994-2013?

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Biological Data 1993-2013

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  • Three days a week randomly selected for biological sampling
  • Boats selected (non-random) and entire catch is studied



port sector date age length sex statarea length.in area
Seward Charter 5261994 11 120.0 F 495938 47.24 resbay
Seward Charter 5261994 NA 128.0 F 475934 50.39 capecleare
Seward Charter 5261994 13 85.0 M 475934 33.46 capecleare
Seward Charter 5261994 13 85.5 M 475934 33.66 capecleare
Seward Charter 5271994 11 126.5 F 485935 49.80 junken
Seward Charter 5271994 11 95.8 F 475934 37.72 capecleare

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Average fish lengths plotted over time


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In the waters around Cape Junken, there is strong evidence of a relationship between mean fish length and time over the years \(1994\) to \(2013\) (p-value\(<0.0001\) from t-stat\(=-5.370\) on \(18\) df).

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The mean fish length in Cape Junken in \(2013\) is estimated to be \(6.8\) inches shorter than the mean fish length in Cape Junken in \(1994\), with a \(95\%\) confidence interval from \(4.35\) to \(9.29\) inches shorter.

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95% CI for change in fish length 1993-2014 in Cape Cleare

OLS-black dashed line (-9.40, 1.37)

WLS-blue line (-10.40, -2.27)

Mixed Effects-red line (-9.35, -0.18)

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Scope of Inference





Technically does not extend to all Seward boats/fish, BUT....




No causal






Questions?

Comparison - Resurrection Bay

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Comparison - Patton Bay

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Quasibinomial Logistic Regression

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  • Each area modeled separately
  • Assumptions
    • Independence
    • Linearity
    • Outliers

Results

There is moderate evidence that the log odds of fishing at Cape Cleare changes over time, assuming a linear trend over years \(1993\) to \(2014\) (p-value\(=0.02479\) from drop-in-deviance F-stat\(=5.89\)).


YEAR coef SE F-stat Pr(>F)
capecleare 0.0404 0.0161 5.891 0.0248
resbay -0.0463 0.0171 7.351 0.0134
nuka 0.0188 0.0385 0.238 0.6309
patton 0.0760 0.0200 12.532 0.0021

Mixed Effects Model

Each area modeled separately



\(length_{i}=\beta_0+\beta_1year_i+\alpha_{i}+\epsilon_{i}\)

\(\epsilon_{i} \overset{iid}{\sim} N(0, \sigma^2)\)
\(\alpha_{i} \overset{iid}{\sim} N(0, \sigma^2_{\alpha})\)

\(i \in (1994, 1995,...,2013)\)

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